Alternating current rectifying element



iatenteol ar. 214, i933 JUHN G. H. LIEBEL, 0F CINCINNATI, OHIO, ASSIGNOR, BY

MESNE ASSIGNMENTS, TO THE UNIQN SWITCH & SIGNAL COMPANY, A CORPORATION OF PENNSYLVANIA ALTERNATING REOZIFYING ELEMENT l lo Drawing. Application filer. April 22,

This invention relates to copper oxide rectifying elements and the process of making the same. Such a rectifier is disclosed in Allen application Serial #164,375. These rectifying elements consist of copper plates, discs, or rings which are heated, preferably in an electric furnace to a temperature of about 1900 Fahrenheit thereby causing the formation on the copper of a layer of red or ruby colored oxide and on top of this a layer of black oxide. After the proper formation of the oxide coating the discs are removed from the furnace and allowed to cool to a temperature somewhat lower than that of the furnace, whereupon they are either quenched in or contacted witha suitable reducing medium, thus converting the surface of the oxide coating into metallic copper. This produces a rectifier of comparatively low electrical resistance in the useful direction and consequently large current capacity, with an absence of variation of its resistance due to contact pressure on the oxide surface and a rectifier which does not require high pressure on the oxide surface to provide a good electrical contact. These discs, plates, or rings can be connected in series, or parallel, or series-parallel, according to the voltage and current requirements to constitute rectifiers suitable for many uses charging storage batteries, for instance. Plates so formed have the characteristic of offering comparatively little electrical resistance to current flow from the reduced surface to the oxide to the copper, and a comparatively high electrical resistance to current flow in the reverse direction.

The object of this invention is to provide rectifying elements of the class described, said elements characterized to a greater degree by a low useful and high inverse resistance.

Another object of the invention is to provide a process of making these elements more uniformly of the desired characteristics above referred to.

Another object is to provide against short circuiting of such elements.

I have found that when examined under a magnification of several hundred di m- 1927. Serial No. 185,911.

eters, a cross-section of many discs produced by prior methods and having high electrical resistance in the useful directlon, that is, from the reduced surface of the oxide to the copper, which they'should not possess for best results and most eiiicient operation. the ruby colored oxide has in parts an uneven, sintered appearance, or if the rectifier be very poor, the ruby colored oxide may have this appearance almost entirely. On discs having good characteristics, the ruby colored oxide has a uniform glassy appearance throughout its entire tion and seems firmly in contact with the copper. In the production of the disc it is very essential that the copper plates be raised to a sufiiciently high temperature to secure the proper oxide formation. If brought to a temperature of only around 1500 Fahrenheit, or lower, the oxide readily scales oil of the copper. If the oxidizing temperature is around 1800 Fahrenheit or higher, no scaling oil will result. In practice a furnace temperature of 1900 Fahrenheit or a little over is used. That is, the temperature is brought as close as practical to the actual melting point of the copper itself (about 1985 Fahrenheit) without actually melting the copper. The ruby colored oxide apparently melts at a temperature somewhat below the melting point of the copper itself. On discs prepared in this manner, if removed from the furnace and allowed to cool and then examined under the microscope the ruby colored oxide will present a uniform glassy appearance and will be firmly adherent to the copper. If, however, the discs are removed from the furnace and quenched in a good reducing medium,

such as transformer oil, while at a high temperature the ruby colored oxide presents to a more or less degree the uneven appearance, this appearance becoming more pronounced the higher the temperature.

In making the reduced surface discs described in the Allen application previously mentioned, after the proper formation of the oxide coating they are allowed to cool to a very dull red heat and then quenched, thereby reducing the surface of the oxide cross-seccoating to metallic copper.

One reason for the sintered or uneven appearance of the ruby colored oxide and its high resistance under these conditions is that the oxide has not yet thoroughly solidified at the time of quenching or contacting of the surface with a reducing medium. It consequently takes the form of a sintered mass, looking something like cinders of all different sizes, instead of the uniform, glassy, firmly adherent type. Another reason probably is that copper oxide at high temperatures has the property of absorbing a considerable volume of gases, which gases are again largely given off on cooling. When cooled normally or even in a moderate blast of air these gases can apparently diffuse out without affecting the physical form of the oxide but if cooled quickly from a high temperature by sudden quenching the gas evolution is so intense that the physical formof the ruby colored oxide is altered and its good electrical contact destroyed.

On discs approximately .015 of an inch thick and up to 1 inches square or round, when quenching in oil it is necessary to have the temperature still within the range of aslightly visible red when viewed by ordinary light, or certainly not much below this, if

SlllllCleIltreduction of the surface coating is to be obtained to give a low resistance plate. If quenched in a solution of alcohol or glycerine with or without water, a lower quenching temperature can be used. A solution of alcohol in oil permits of a lower quenching temperature than oil alone.

One method of overcoming, at least partially, the difficulties mentioned is that the discs or plates are placed in the furnace in an air or oxygen atmosphere at a temperature of around l900 Fahrenheit, and maintained at that temperature for a sufficient period of time to produce the desired thickness of oxide coating. After the plates have been properly oxidized they are removed from the furnace and allowed to cool to about room temperature.

After the discs have cooled they are again placed in an electric furnace, brought up to the proper temperature and then quenched. The proper quenching temperature depends upon the particular quenching medium used, the dimensions of the discs, the number of them quenched at one time, how closely they are spaced apart. and the degree of reduction of the surface coating desired. For instance, for a fairly low resistance disc of about .015 of an inch thick and 1- inches square which'is to be quenched in trans former oil, a few at a time, a temperature of about 1450 Fahrenheit is satisfactory. For a of an inch thick, 2 inches diameter disc, a uenching temperature of around 1200 Fa renheit is satisfactory. When a good number of discs are heated up and quenched simultaneously and are suspended in the furnace not too far apart, so that there'is not much possibility of the discs cooling in the short period of time between removal from the furnace and the quenching, I find that the best quenching temperature is from 50 to 150 Fahrenheit lower than the above values, depending on the number of plates, how close they are together, etc., with a transformer oil quenching medium.

It is obvious that in the method outlined in the Allen application previously mentioned, instead of removing the discs from the furnace and then quenching when they have cooled to exactly the right amount, the furnace temperature can be reduced to the desired temperature for quenching provided the cooling time of the furnace is not too long, or that they could be removed to another and cooler part of the same furnace, or transferred from the oxidizing furnace to another furnace which is maintained at a lower temperature. I have found, however, that if the discs are allowed to cool down to room temperature and then reheated to the proper quenching temperature, their performance is vastly improved over that obtained by simply bringing down from the oxidizing to the quenching temperature in one step. This may be due to the causes previously outlined.

I have noted one additional contributing factor. In producing the smaller size discs, plates, or rings, as previously mentioned, their edges have an opportunity of cooling somewhat between the time they are removed from the furnace and the time they are actually quenched. Such plates show a rim of black unreduced oxide around the edges. If, however, this process is carried out with discs say of an inch thick and 2 inches or more in diameter, or even in some cases on of an inch thick and 1 inches diameter, on account of the larger mass of copper the edges do not cool appreciably more than the rest of the disc or plate, and consequently the surface is more or less reduced around these edges and the creepage around these edges allows a high inverse current to flow or even may amountto a partial short circuit on the inverse direction of current flow. When discs are made double, that is, placed in the furnace back to back and close enough together so that only one side of each disc becomes oxidized, this reduction runs around the edge and back onto the unoxidized surface. It would seem that this could be prevented by making discs which are coated on both sides, but this is not the case as the oxide cracks off at the edges on cooling, and thereby permits the reduced surface to short circuit towards the exposed bare copper.

I have found that if the disc is removed from the oxidizing furnace after proper formation of the oxide coating, and 1s then allowed to cool to room temperature, and

then subseqpently again placed in a furnace a moderate amount above room temperature the oxide coating on the edges cracks ofi, a1- lowing a thin roughened edge of the black oxide coating on top at the edges of the plate. Then when reheated, during the short time interval of removal from the furnace and the actual quenchin operation, these loosened rough edges 0 the black oxide have an opportunity of cooling down for asmall distance from the riphery of the disc to a temperature be ow that at which reduction of the oxide to copper takes place. If prior to the reheat and quenching operation the discs are examined, and the oxide at the edges is thoroughly cracked off either by means of coarse sandpaper'or emery cloth or light blows from a tool, the results obtainable are still further improved. Rectifier plates made by this method in the smaller dimensions and thicknesses of plate say up to 1 inches square or equivalent area, and from .015 to .030 of an inch thick can be produced commercially with very good characteristics.

Where the best possible characterlstics are desired, especially a low inverse current, or where plates of larger capacity are desired than those that can be produced by the foregoing method, I emplo another means of preventing this edge e ect or creepa ev of inverse current around the edges 0- the plates, discs, or rings, which provides extraordinarily uniform and satisfactory results, and which is excellently adapted to quantity production. The same method as previously outlined is carried out, removing the plates from the oxidizing furnace, allowing them to come to room temperature, then chipping off any remaining oxide coating at the edges. Then before they are placed in the reheat furnace I coat the edges with an enamel or glaze which will become fused in the reheat furnace and form a tough adherent insulating layer around the ed es of the disc. The disc is then reduced in t e usual manner and then presents the appearance of an insulating rim around the edges with the center part reduced to metallic copper. A great variety 'of glazing materials is possible. Any glazing material which is sufliciently electrically insulating, which is not affected by the reducing process and which has a thermal expansion coefficient sufiiciently close to that of metallic copper or the copper oxides will answer the purpose. F have found a mixture of equal arts by wei ht of red lead and finely powdiered fused orax satisfactory. The percentages can vary considerabl and still produce satisfactory results. he powder can be mixed with a small quantity of any suitable medium such as oil or water, to make it adhere sufliciently until fusion begins.

In carrying out the foregoing process it is also possible instead of allowing the plates to come down to room temperature and then reheating, to coat the edges -of the plates with the glaze prior to putting the plates in the oxidizing furnace. For this purpose I have found a satisfactory mixture consisting of approximately equal arts by wei ht, of red lead, powdered fused orax, and nely powdered quartz mixed with enough of any suitable material to make it adhere until fused as previously outlined. By this method it is possible withoutallowing the plates to cool, to remove them from the oxidizing furnace to another furnace maintained at the proper quenching temperature, or to another art of the-oxidizing furnace which is malntained at the proper quenching temperature, without the intermediate cooling and handling operations. Plates made in this manner come -out similar to those previously described.

Another method by which the same result is accomplished is to coat the edges of the plates with a material, for instance, red lead, which will combine with the oxide around the edges and form a new compound or mixture of materials around the edges which is firmly adherent, which will I withstand the quenchingoperatiomand which will have high electrical resistance. The red lead can be mixed with just enough oil or other fluid to make it possible to very thinly coat the edges of the plate, prior to placing it in the oxidizing furnace. It is very desirable that only a very small amount of the material be used as just a small amount does the work and permits the high resistance layer to run in about an eighth of an inch or so from the edge, whereas if too much is used it will run in over the entire surface of the plate while the oxide is being formed. Plates made in this manner also have an appearance similar to those previously described but do not present a glazed coating at the edges, but rather a continuation of the oxide coating as it appears before the quenching and reduction operation, which insulating rim with the red.

While I have described the process of producing a glazed or high resistance 11111 more particularly in connection with a redueed surface type of copper oxide rectifier, this portion of my invention is equally a plicable to copper oxide rectifier elements w era the surface of the oxide isnot reduoea, and Itherefore do not wish to be limited to' the reduced surface type of elem ment only.

- Having described my invention, I claim:

1. The process of making a rectifying element comprising coating the edges of a-copr plate with a non-conducting material adapted to adhere thereto, coatin the center of said plate with copper oxi e and reducing the surface of said coating to metallic copper.

2. A copper oxide rectiffiing element" comprising a copper member aving an inwardly' lying surface of copper oxide and an edge surface of insulating glaze other than a copper derivative.

3. The process of making a copper oxide 5 rectifying element which comprises heating a copper member to produce a coating of co per oxide thereon, cooling said member su stantially to room temperature, applying to an edge of said member material adapted to form an insulating glaze u on heating, reheating said element an thereby formin the glaze, and then reducing the inward y lying copper oxide coating to copper at a temperature below the melting 85 point of red copper oxide but not substantially below 1050 F.

4. The process of making a copper oxide rectifying element which comprises heating a copper member to produce a coating of copper oxide thereon, cooling said member su stantially to room temperature, a p1 ing to an edge of said member materiafad apted to form, in combination with the oxide on said edge, an insulating glaze upon heating, reheating said element and thereby forming the glaze, and then reducing the inwardly lying copper oxide coatingto copper at a temperature below the melting point of red copper oxide but not substantiallybelow 105Q F. I

5. The process of making a copper oxide rectifying element comprising coatin an edge of said element with material adapted to form an insulating glaze thereon upon heating, and then properly heating said element to oxidize an inwardly lying surface thereof and properly reducing said surface.

6. The process of making rectifying elements of the class described comprising the step of oxidizing one surface of a co per element at a temperature just below t e melting point of copper, cooling said element su stantially to room temperature, then bringing said element to a temperature below the melting point of red copper oxide my name.

J OHN G. H. LIEBEL. 

